This invention was made while the inventors were employed by Nova Sensor, Fremont, Calif. The patent application shall be assigned Schlumberger Industries, Ltd. pursuant to contractural obligations.
1. Field of the Invention
The invention relates to electromechanical sensors and more particularly to semiconductor sensors with overpressure protection.
2. Description of the Related Art
Silicon possesses well-known electrical and mechanical properties that are used in an ever-increasing variety of applications directed to the detection and measurement of the dynamics of physical processes. Many of the important electrical and mechanical qualities of silicon are explained by Kurt E. Petersen in "Silicon as a Mechanical Material", Proceedings of the IEEE, Vo. 70, No. 5, May 1982, which is incorporated herein by this reference.
In particular, the resistance value of a silicon resistor changes in response to a flexing of the silicon crystal. This piezoresistive response has been used to produce silicon-based pressure transducers. For example, a change in the differential pressure applied across a silicon diaphragm in which a resistor is formed causes a change in the resistor value. The change in the resistor value is approximately proportional to the change in differential pressure. Thus, measurement of the resistor value can be used to determine the differential pressure applied across the diaphragm.
In practice, changes in an individual resistor's value can be small and difficult to measure. Therefore, in a typical silicon diaphragm, four resistors are formed in a Wheatstone Bridge configuration. Even small changes in individual resistor values can contribute to a significant offset in the Bridge and can provide an easily detectable signal used to measure resistor value change.
In addition to silicon's piezoresistive qualities, it possess exceptional strength. It has an elastic modulus comparable to that of steel. Silicon's strength is most advantageous at microscopic size where it results in the considerable flexibility and resiliency of silicon microstructures.
Since silicon is a single crystal material, it is more resistant to cyclic mechanical stresses than are polycrystalline metals. It can be stressed repeatedly with little effect. Thus, it suffers virtually no measurable hysteresis or work hardening over time.
Devices incorporating silicon pressure sensors ordinarily are constructed so as to permit the silicon pressure sensor to interface with a measurand environment without contamination of the sensor. The sensor could be rendered inoperable by such contamination.
In some environments, a silicon pressure sensor can be adequately protected from environmental contaminants by coating it with a silicone gel. Silicones are chemically inert polymers containing chains of silicon atoms, in contrast to organic polymers which contain chains of carbon atoms. However, this approach can be insufficient in harsh environments. For example, a silicone gel provides inadequate protection in some process control applications which require long-term direct contact between a sensor and potentially corrosive fluids.
One earlier solution to the contamination problem in hostile environments is embodied in the exemplary sensor device 20 illustrated in FIG. 1. In essence, the device 20 isolates its semiconductor differential pressure sensor 25 from hostile measurand environments through the use of thin isolation diaphragms 22 and 24 which interface directly with the measurand environments. The pressure sensor 25 is sealed within a housing 27 and is mechanically coupled to each of the two isolation diaphragms 22 and 24 through an inert substantially incompressible fill material such as silicone oil. The oil completely fills each of a pair of discrete tubular conduits 28 and 30. A first conduit 28 communicates directly with one side of the pressure sensor 25 and with a first isolation diaphragm 22. A second conduit 30 communicates directly with the opposite side of the pressure sensor 25 and with a second isolation diaphragm 24.
In operation, the two isolation diaphragms 22 and 24 are exposed to two distinct measurand environments for which a pressure differential is to be measured A pressure difference between the two measurand environments causes a displacement of at least one of the two isolation diaphragms 22 and 24 which, in turn, leads to a pressure difference between the silicone oil volumes in the respective tubes 28 and 30. The result is a measurable differential pressure across the silicon differential pressure sensor 25.
While earlier silicon differential pressure sensors suitable for use with devices such as that shown in FIG. 1 have been acceptable, there have been shortcomings with their use. One such shortcoming stems from the need to protect such sensors from overpressure conditions. A pressure sensor could be damaged if exposed to an excessive differential pressure.
An overpressure condition could result, for example, if one of the two isolation diaphragms 22 or 24 was accidently exposed to the full static pressure of one of the two measurand environments. In practice, for example, the static pressure of the two measurand environments each might be on the order of 3000 pounds per square inch (PSI) while the pressure difference between the two measurand environments might be on the order of 10 PSI. Unfortunately, the semiconductor pressure sensor 25 could be damaged if the full static pressure was applied differentially across it. Consequently, precautions must be taken to prevent such damage.
In the past, such precautions generally involved schemes to prevent the exposure of a semiconductor pressure sensor to the full overpressure. One earlier scheme, illustrated in FIG. 1, involved the use of stop surfaces 32 and 34 which limited the range of motion of the isolation diaphragms 22 and 24. Even during exposure of one of the two isolation diaphragms to extreme overpressures, the displacement of the exposed isolation diaphragm would be limited by its corresponding stop surface. Therefore, the pressure sensor 25 would not be exposed to the full overpressure.
Earlier protection schemes, such as the two described above, generally have functioned but have been costly and difficult to manufacture. One reason is that such schemes typically require the manufacture of precision components used to prevent a silicon pressure sensor from being exposed to an overpressure condition. While these components have been used successfully to protect semiconductor pressure sensors from overpressure conditions, they often are expensive to produce and add significantly to overall cost of a device.
Thus, there has been a need for a silicon pressure sensor which can withstand overpressure conditions. The present invention meets this need.